A conventional image forming device using an electrographic method forms an electrostatic latent image that corresponds to image information by exposing a surface of a photosensitive drum using an exposure head, such as a light emitting diode (LED) and the like after uniformly charging the surface of the photosensitive drum by a charging roller. Then, a toner image is formed by electrostatically attaching a thin layer of toner on a development roller to the electrostatic image. After transferring the toner image onto a sheet carried by a carrying belt using a transfer roller, an image is formed on the sheet by fixing the toner image using a fuser.
This type of image forming device uses a belt heating type fuser. In such a fuser, a fusion belt formed by an endless belt is heated, and a fusion roller is pressed by a pressure application roller facing across the fusion belt, thereby forming a nip part. The carried sheet is pinched by the nip part, and the toner image is fixed onto the sheet by heat and pressure. See Japanese Laid-Open Patent Application No. 2009-151115 (paragraphs 0012-0020, 0028 and FIG. 1).
However, in the above-described conventional technology, because the toner image is fixed onto the sheet by pinching the sheet that has been carried, by the nip part formed by pressing the fusion roller with the pressure application roller facing across the fusion belt, there is a problem that excess reverse curling occurs on the sheet after the fusion if a temperature difference between the pressure application roller and the fusion belt and fusion roller is large at the time of fusion.
Such a sheet with a large amount of reverse curling causes carrying ability of the sheet after fusion and stackability of the sheet on a stacker to be reduced. The present application is made in consideration of solving the above-described problem and has an object to provide a device that suppresses the reverse curling amount at the time of fusion at the fuser.